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Research
Epitaxial growth of oxide based semiconductors
Transparent conducting oxides (TCOs) can be used for high-power and transparent electronics and are proposed candidates for back-end-of-line (BEOL) compatible vertical complementary metal-oxide semiconductor (CMOS) channel materials.
Currently, the oxide MBE group at Bremen University (also: see the MBE-by-Design subgroup) investigates the growth and materials properties of n-type (Al,Ga)2O3 and n-type (In,Ga)2O3 in their different polymorphs (corundum, monoclinic, orthorhombic). The group further studies oxide-nitride hybrids (e.g., Ga2O3-AlN and AlN-Ga2O3 heterostructures) to realize 2-dimensional electron gases (2DEGs) at their interfaces. High-mobility p-type TCOs will enable complementary transistor solutions, providing more flexibility to design and implement more efficient BEOL vertical CMOS devices—and are also being investigated in the oxide MBE group at Bremen University.
To synthesize n-type and p-type TCOs, the group uses three different MBE variants: (i) conventional MBE, suboxide MBE (S-MBE), and metal-oxide-catalyzed epitaxy (MOCATAXY). Using three different MBE variants allows to extent the kinetic and thermodynamic conditions under which TCOs may be nucleated and grown in the MBE reactor.
Two-dimensional semiconductors
Two-dimensional (2D), atomaically thin layers exhibit special properties such as high conductivity and mechanical stability for graphene and a direct band gap for some transition metal dichalcogenides such as MoS2. The optical properties of exfoliated semiconductor crystals such as MoS2, WS2 , MoSe2 and WSe2 will be investigated as well as the possibility to manipulate the layer properties by laser irradiation.
A further objective is the growth of 2D materials by atomic layer deposition (ALD). A focus is the growth of mono- and multilayers of MoS2 and WS2. Furthermore, by the realization of ternary MoWS2 as well as the growth of heterostructures from these 2D materials it is expected to manipulate and control the optical properties.